http://www.couchsurfing.org/
CouchSurfing is your ticket to explore the world -- from the road or from your own home. Over a million members are out there waiting to learn about your culture and show you some of theirs. Eat borscht in Odessa. Watch kangaroos in Adelaide. Find the best noodles in Tokyo. Show travelers the best live music near your home, or your favorite hiking trail. Meet locals in your own city who share your interest in travel and the world.
http://www.meetup.com/
Meetup is an online social networking portal that facilitates offline group meetings in various localities around the world. Meetup allows members to find and join groups unified by a common interest, such as politics, books, games, movies, health, pets, careers or hobbies. Users enter their ZIP code (or their city outside the United States) and the topic they want to meet about, and the website helps them arrange a place and time to meet.